Neoprene has been widely used as a base for adhesives in formulations intended to take advantage of its outstanding characteristics, in particular, its high bond strength, flexibility, and resistance to attack by oils, chemicals, water, heat, sunlight, and ozone. Most of the prior neoprene adhesive formulations have used an organic solvent medium. Owing to concerns regarding the toxicity and flammability of solvents as well as their adverse effects on the environment, emphasis has now shifted toward development of water-based neoprene latex adhesives.
Numerous formulations of neoprene latex adhesives have become available as described in a DuPont bulletin entitled "Neoprene Latex-based Adhesives" by D. G. Cole (printed October 1991). All of the neoprene latexes described in this publication are anionic in character, except for DuPont Latex 115, which is nonionic. Latex 115 is a copolymer of chloroprene and methacrylic acid, dispersed and stabilized with polyvinyl alcohol. The presence of carboxyl groups in the polymer of this material gives it a high reactivity for undergoing crosslinking reactions with bivalent metal oxides at room temperature. This provides important advantages such as improved hot bond strength as compared to anionic latexes. Superior mechanical and chemical stability are also distinguishing features of Latex 115. As pointed out in the referenced bulletin, Latex 115 behaves in a totally different manner from anionic latexes; thus measures which are required or useful in compounding for Latex 115 differ substantially from those for other types of neoprene latexes.
The product bulletin gives various directions and suggestions for compounding of adhesive formulations based on Latex 115. No disclosure is given, however, regarding preparation of a Latex 115 formulation that would be amenable to application by aerosol spraying. An aerosol-sprayable nonionic latex adhesive is desired to enable easier application and to obtain a higher yield in terms of coverage per unit amount of latex. Spraying of adhesive formulations based on Latex 115 has presented difficulty owing to the tendency of this material to undergo agglomeration and coagulation, resulting in rapid plugging of spray orifices.
Another DuPont bulletin entitled "Contact Adhesives Based on Carboxylated Neoprene Latex 115" by D. G. Cole and J. W. McDonald discloses various contact adhesive formulations using this form of neoprene and methods of preparing the formulations. This reference, however, is not concerned with sprayable adhesives, and use of an alkali reagent such as sodium hydroxide is recommended to enable larger amounts of zinc oxide to be included. The presence of alkali is also said to enable preparation of the adhesive by a "one-shot high-shear mixing" technique. Avoidance of the use of a caustic alkali would be desirable owing to the hazardous nature of such materials. This reference further teaches use of coalescence aids and thickeners not appropriate for a sprayable material. No disclosure is given regarding surfactants.
One of the most important features of sprayable water-based adhesives is the drying time of the sprayed-on adhesive. A minimized time is desirable, particularly for manufacturing applications where prolonged drying time would increase costs by slowing up production. It is therefore desirable to use a formulation method which provides for effective mixing and sprayability, while requiring a minimum amount of water.
Measures taken to enhance sprayability of water-based latex paints and adhesives are disclosed in various prior patents. U.S. Pat. No. 4,384,661, issued on May 24, 1983, to Page et al., discloses use of a nonionic surfactant having a high HLB value to avoid foaming in latex paint polymer systems. Aerosolsprayable latex paints using polymer mixtures or alcohol additives are disclosed in U.S. Pat. No. 4,420,575, issued on Dec. 13, 1983, to Rapaport et al., and U.S. Pat. No. 4,265,797, issued on May 5, 1981, to Suk. U.S. Pat. No. 4,477,613, issued on Oct. 16, 1984, to Evans et al., discloses stabilization of an aqueous-based tackifier by providing a minor amount of a nonionic or anionic surfactant and a minor amount of an elastomeric latex. U.S. Pat. No. 4,004,049, issued on Jan. 18, 1977, to Horwat et al., discloses a sprayable latex adhesive provided with a controlled degree of instability to obtain improved adhesion. None of these patents discloses an aerosol-sprayable water-based adhesive based on a nonionic neoprene latex polymer.